Proximity sensor

Dear net community,
I would like to build a proximity sensor for a Motorhome that could trigger
an alarm in various ways.
What I want is to be able to set off an alarm inside the vehicle if it is
approached during the night.
PIRs wouldn't be any use because they may be set off by animals. What I was
thinking of was something that would sense the presence of a body at about
chest level, but not ground level, around the perimeter of the vehicle. If
it were possible to switch this circuit so that at other times the alarm
would be triggered if the door handles were operated that would be a bonus.
The output of this would also need to be switchable between a small buzzer
type of warning device and a loud siren depending on how the circuit was
set. The working voltage would be 12v dc.

I have to say now that my knowledge of electronics is basic although I do
have experience. Any help with either a circuit diagram, suggestions as to
how I might obtain one or any books that may be of help would be most
welcome.

Regards
Steve
England